India's opposition said Sunday it would seek tougher punishments for juveniles after the first verdict in the New Delhi gang-rape case saw a teenager sentenced to three years' detention, sparking widespread anger.

The rape and murder of a 23-year-old student by six attackers on a moving bus last December sparked nationwide protests and led to reforms that mandated longer sentences for adult sex offenders.

Sushma Swaraj, opposition leader in the lower house of parliament, said she would introduce a bill this week to amend the law for juveniles.

"This meagre punishment of just three years does not do justice," Swaraj wrote on Twitter.

"The sentence must commensurate with the gravity of the offence irrespective of the age of the offender," she added.

On Saturday a juvenile court in New Delhi sentenced the only under-age suspect in the gang -- who was 17 at the time of the crime -- to three years in a correctional facility.

This was the maximum sentence under Indian law, which treats all under-18s as children and seeks to reform rather than punish them.

"TRAVESTY: December 16 teen rapist 'gets away' with murder," a headline in the tabloid Mail Today read, summing up the mood.

The convicted teen will spend about 28 months in a juvenile detention centre, having already spent about eight months in custody awaiting the verdict.

"He can watch TV, play games while doing time," the Hindustan Times reported, while pointing out that police sources had earlier described the teenager as "the most brutal" of the six attackers.

The Times of India said the gang-rape victim had "been denied justice" by the juvenile court.

Subramanian Swamy, a politician from the main opposition Bharatiya Janata Party, told AFP the teenager "should have been executed" and he intended to file an appeal against Saturday's court order.

Swamy has already lodged a petition in the Supreme Court challenging India's juvenile law for not taking the gravity of a crime into account during sentencing.

"It's ridiculous to think you can reform a person who has committed a heinous crime, who has raped and murdered a young woman in such a brutal fashion," he added.

According to the teenager's defence lawyer, his conduct will be observed and the sentence could be reduced for good behaviour.

The juvenile was employed to clean the bus where the attack took place and often slept rough or inside the vehicle, reports say.

A child rights activist who knows him said he grew up poor in a village in the northern state of Uttar Pradesh and moved to Delhi on his own at the age of 11 when he began a string of menial jobs.

"He changed jobs all the time, desperate to earn more and send money to his family," the activist told AFP on condition of anonymity.

A government panel set up after the gang-rape to recommend changes to sex crime laws rejected calls to lower the age at which people can be tried as adults from 18 to 16.

The panel's report in January also said it was "completely dissatisfied with the operation of children's institutions", a view echoed by several child rights activists.

Shahbaz Khan, co-founder of Haq: Centre for Child Rights, which has provided counselling to the convicted juvenile told AFP there were often "no trained social workers and psychologists placed inside these institutions", thus undermining any effort at rehabilitation.

The attack on the young woman brought simmering anger about endemic sex crime in India to the boil, and turned her attackers into public hate figures.

But despite soul-searching and a new law toughening sentences for rapists, sex crimes have continued unabated, with almost every day bringing news of a new grave offence.

News emerged Saturday evening of another attack in the Noida suburb of the capital, where a woman was allegedly gang-raped by five men including two police constables.

The Press Trust of India said the 25-year-old victim was attacked while visiting a male friend, who was also assaulted by the gang of five.

Last month a 22-year-old photographer was gang-raped in Mumbai while taking pictures at an abandoned mill in a posh part of the commercial capital.

Protesters outside the juvenile court Saturday and the victim's family called for the teenager to be hanged.

The victim, a physiotherapy student, died of internal injuries two weeks after being raped and assaulted with an iron bar on the night of December 16.

Her male companion was beaten up before both were thrown bleeding from the bus.

A separate trial of the four adult suspects in a fast-track court is hearing closing arguments and is expected to wrap up in the next few weeks, with the men facing a possible death sentence if convicted.

The fifth adult, the suspected ringleader, died in jail in an apparent suicide.